This invention relates generally to integrated circuits, and more particularly to integrated circuit packaging.
The term "integrated circuit" usually refers to an integrated circuit die and its associated package. A common way to package an integrated circuit die is to attach it to a lead frame and then to encapsulate it within plastic. A typical lead frame includes a die attach area and a number of bonding fingers which are electrically coupled to the die by bonding wires.
Integrated circuit dies are being designed with smaller and smaller feature sizes to increase the performance and to reduce the cost of the integrated circuit. An integrated circuit die having smaller feature sizes will usually end up being smaller than a functionally equivalent integrated circuit die having larger feature sizes. While integrated circuit die tend to be shrinking, integrated circuit package sizes are commonly fixed by standards and conventions.
A yield problem occurs when small die are packaged within lead frames designed for larger die. Since the die is physically smaller, longer bonding wires are required to electrically connect the die to the bonding fingers of the lead frame. These long bonding wires can be quite flexible and have the tendency to short out with adjacent wires due to an intrinsic twist of the bonding wires or during the encapsulation process.
One solution to this problem is to make lead frames which accommodate smaller die sizes. This tends to be an expensive solution because the bonding fingers of the lead frame become so small and so closely spaced that the lead frames can no longer be economically stamped out but, rather, must be manufactured by an expensive etching process.
Another solution to this problem is to use what is known as an interposer. With an interposer, a number of intermediate bonding islands are provided on the die attach pad of the lead frame so that the die can be wired to the bonding fingers in two jumps. This is accomplished by wiring the bonding pads of the die to the bonding islands of the interposer with a first (inner) set of wires, and wiring the bonding islands to the bonding fingers of the lead frame with a second (outer) set of wires. By making the connection between the die and the bonding fingers in two jumps, the long, direct connection wires have been replaced by two sets of shorter wires. These shorter wires are less likely to short against adjacent wires than their longer counterparts.
Prior art interposers are essentially small printed circuit boards having a relatively thick, rigid, insulating substrate upon which the conductive islands are formed. The printed circuit boards are attached to the die attach pad of the lead frame, and are provided with a central aperture to accommodate the attachment of an integrated circuit die through the interposer to the die attach pad.
While the prior art interposers perform their function admirably, they do have an number of drawbacks. First, they are relatively expensive devices because they are, essentially, miniature printed circuit boards. Also, since the size of the die aperture is fixed, a particular interposer can only be used for a small range of die sizes. Obviously, an interposer cannot be effectively used when the die size is larger than the size of the die aperture. If an interposer was used with a die much smaller than the size of the die aperture the inner set of wires must be made longer, reducing the effectiveness of the interposer. Another drawback with prior art interposers is that the bonding islands have an intrinsic inductance which can degrade high-frequency performance of the integrated circuit.
The prior art does not, therefore, disclose an interposer for integrated circuit packages which increases the yield of functional integrated circuits, which is economical to produce, which has superior high-frequency performance, and which can accommodate integrated circuit die having a range of sizes.